There are known wireless power supply systems for supplying electric power and charging batteries mounted on electric vehicles via a wireless connection without use of plugs for power supply. A wireless power supply system includes a power transmission coil provided in a power transmission device on the ground side and a power receiving coil provided in a power receiving device mounted on a vehicle and facing the power transmission coil so as to supply electric power from the power transmission coil to the power receiving coil via a wireless connection. The wireless power supply system is required to regulate the electric power output from the power transmission device depending on a state of charge of a battery installed in the power receiving device. Patent Literature 1 describes such a system configured to transmit a signal of a power supply command from a power receiving device to a power transmission device and select one of power supply devices based on the transmitted signal.